Subchannel feedback for OFDMA systems

ABSTRACT

A method of sub-channel feedback in OFDMA systems is provided. A wireless receiving device (STA) receives a radio signal from a transmitting device (AP) over a wide channel in an OFDMA system. The radio signal is transmitted over multiple sub-channels of the wide channel. The STA estimates channel quality information based on the received radio signal for each sub-channel. The STA then sends feedback information to the transmitting device. The feedback information comprises the estimated channel quality information for a selected subset of sub-channels from the wide channel based on a predefined rule. In one embodiment, the feedback information is embedded within an ACK/BA frame or is carried in a frame immediately subsequent to the ACK/BA frame.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 from U.S.Provisional Application No. 61/947,682, entitled “Sub Channel Feedbackfor OFDMA System,” filed on Mar. 4, 2014, the subject matter of which isincorporated herein by reference.

TECHNICAL FIELD

The disclosed embodiments relate generally to wireless networkcommunications, and, more particularly, to feedback process in wirelesscommunications systems.

BACKGROUND

In IEEE 802.11 wireless systems, channel sounding and feedback processis commonly used for channel estimation. In MIMO systems, each channelsounding and feedback process is followed by a series of MIMO frameexchange. During channel sounding and feedback, a transmitting device(initiator) sends a sounding announcement (e.g., null data packetannouncement (NDPA)) followed by a sounding packet (e.g., null datapacket (NPD)) to a receiving device (responder) participating in theprocess. The responder estimates the channel during the preamble portionof the sounding packet. The responder then feedbacks the average SNR(signal-to-noise ratio) and CSI (channel state information) to allow theinitiator to compute the transmit antenna (precoding) weights for MIMOtransmission. Feedback packet may also include other channel qualitymetrics such as MCS, BER, SNR/SINR, and mutual information.

Feedback of accurate channel quality information such as SNR and MCSallows the transmitter to make correct decision regarding transmissionbandwidth adjustment as well as MCS adaptation to improve systemperformance. In current implementation, channel quality information isprovided based on a fixed sub-channel (e.g., the sounding bandwidth) andobtained through a sounding and feedback protocol. The channelconditions, however, could be significantly different in differentsub-channels due to frequency selective fading. To have channel qualityinformation for all sub-channels, multiple requests and feedbacks arerequired. This leads to increased system overhead and channelcontention.

In orthogonal frequency division multiple access (OFDMA) systems,frequency division multiple access is achieved by assigning differentOFDM sub-channels to different users. OFDMA design can benefit frommultiuser diversity gain. CSI feedback for multiple users are requiredto achieve multiuser diversity gain. However, the existing CSI/SNRfeedback scheme does not work due to large feedback overhead. Forexample, the compressed CSI feedback has very large overhead. On theother hand, SNR feedback only provides feedback for average SNR overdata subcarriers and space-time streams. It cannot provide SNR onsub-channels.

A solution is sought to provide sub-channel SNR feedback and CSIreporting with reduced overhead and channel contention.

SUMMARY

A method of sub-channel feedback in OFDMA systems is provided. Awireless receiving device (STA) receives a radio signal from atransmitting device (AP) over a wide channel in an OFDMA system. Theradio signal is transmitted over multiple sub-channels of the widechannel. The STA estimates channel quality information based on thereceived radio signal for each sub-channel. The STA then sends feedbackinformation to the transmitting device. The feedback informationcomprises the estimated channel quality information for a selectedsubset of sub-channels from the wide channel based on a predefined rule.The channel quality information comprises an average signal to noiseratio (SNR) of a selected sub-channel, or a sum throughput of allspatial streams of the selected sub-channel. In one embodiment, theselected subset of sub-channels is based on a predefined channel qualitythreshold. In another embodiment, the selected subset of sub-channels isbased on a predefined number of sub-channels having the best channelquality. In yet another embodiment, the feedback information is embeddedwithin an ACK/BA frame or is carried in a frame immediately subsequentto the ACK/BA frame.

Other embodiments and advantages are described in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a channel sounding and feedback process withsub-channel SNR feedback in a wireless system in accordance with onenovel aspect.

FIG. 2 illustrates a wireless system with sub-channel SNR feedback inaccordance with one novel aspect.

FIG. 3 illustrates a novel feedback frame format in an IEEE 802.11acwireless system.

FIG. 4 illustrates transmission sub-channels in a wireless system andexamples of sub-channel SNR feedback.

FIG. 5 illustrates the theoretical capacity in an OFDM/OFDMA wirelesssystem with SNR sub-channel feedback.

FIG. 6 illustrates SNR sub-channel feedback in OFDMA systems supportingMU-MIMO.

FIG. 7 illustrates a two-step SNR sub-channel feedback method in OFDMAMU-MIMO systems.

FIG. 8 illustrates a novel acknowledgement frame and a novel blockacknowledgement frame with piggybacked SNR sub-channel feedback.

FIG. 9 illustrates one embodiment of a SNR sub-channel feedback frameafter ACK/BA frame.

FIG. 10 illustrates another embodiment of SNR sub-channel feedback inresponse to polling.

FIG. 11 illustrates one embodiment of SNR sub-channel feedback afterpolling and OFDMA data transmission.

FIG. 12 illustrates another embodiment of piggybacked SNR sub-channelfeedback after OFDMA data transmission

FIG. 13 is a flow chart of a method of transmitting SNR sub-channelfeedback in a wireless communication system.

FIG. 14 is a flow chart of a method of receiving SNR sub-channelfeedback in a wireless communication system.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 illustrates a channel sounding and feedback process withsub-channel SNR feedback in a wireless system 100 in accordance with onenovel aspect. Wireless system 100 comprises a transmitting device (e.g.,an access point AP) and a plurality of receiving devices (e.g., non-APstations STA1, STA2, and STA3). During channel sounding 101,transmitting device AP sends a sounding announcement (e.g., null datapacket announcement (NDPA) not shown) followed by a sounding packet(e.g., null data packet (NDP) not shown) to receiving devices STA1-STA3.The NDPA is transmitted first to inform the intended receiving device(e.g., via STA INFO fields) and the NDP is then transmitted for theintended receiving devices to estimate the channel. For example, in theNDP, the L-STF, L-LTF, L-SIG fields are used for setting up theprotection field against legacy devices. The signaling information forthe NDP signal is carried in the VHT-SIG-A and the channel estimation isperformed on the LTFs (long training fields). After channel sounding101, transmitting device AP also sends a downlink (DL) data packet 102to STA1. Typically, the channel sounding 101 is sent over a widechannel. In an OFDMA system, the DL data packet 102 is sent to multipleusers over different sub-channels.

Channel sounding and feedback is a procedure to support transmitbeamforming and fast link adaptation. Feedback on accurate channelquality information such as accurate SNR/MCS information allows thetransmitter to make correct decision regarding the transmissionbandwidth as well as MCS adaptation to improve system performance.Moreover, in OFDMA systems, CSI (channel state information) feedbackfrom multiple users are required to achieve multiuser diversity gain.

In the current IEEE 802.11 implementation, the feedback information isprovided based on the current channel bandwidth. For example, SNR/MCSfeedback is provided for either 20 MHz or 40 MHz depending on thesounding packet. If the sounding signal occupies 40 MHz, then theSNR/MCS feedback is provided for 40 MHz. The same SNR/MCS feedback isassumed valid for 20 MHz sub-channel. Such approach is not sufficientfor wireless systems where the transmission bandwidth can range from 160MHz to 20 MHz. This is because the channel conditions could besignificantly different in different sub-channels due to frequencyselective fading. In OFDMA systems, a sub-channel bandwidth is around 2MHz. As a result, there will be 40 sub-channels for 80 MHz WLAN widechannel and 80 sub-channels for 160 MHz WLAN wide channel. To have SNRinformation for all sub-channels, multiple requests and feedbacks arerequired. This leads to increased system overhead and channelcontention.

In one novel aspect, sub-channel SNR feedback is proposed to facilitateand to improve the performance in OFDMA systems. The receiver providesSNR feedback information for a subset of all the sub-channels based on apredefined condition. Different feedback schemes are also proposed. In afirst embodiment, a piggybacked feedback scheme is proposed. Forexample, STA1 sends an acknowledgement (ACK) or block ACK (BA) frame 111in response to the DL data packet 102. The SNR sub-channel feedbackinformation is embedded within the ACK/BA frame 111 to reduce channelcontention. In a second embodiment, AP-polled feedback scheme is used.For example, AP sends a polling frame 103. In response, STA2 sends afeedback frame 121 back to the AP. In a third embodiment, unsolicitedfeedback scheme is used. For example, STA3 simply sends a feedback frame131 to the AP without solicitation. Note that although channel sounding(101) is illustrated in FIG. 1 for channel estimation purpose, LTFs forchannel sounding can be sent after OFDMA format data traffic (e.g., DLDATA 102). It is thus possible for the STAs to provide feedbackinformation without a separate channel sounding procedure.

FIG. 2 illustrates a wireless system 200 with sub-channel SNR feedbackin accordance with one novel aspect. Wireless system 200 comprises atransmitting device 201 and a receiving device 211 communicating witheach other via a wireless SU-MIMO channel 221. Transmitting device 201comprises memory 202, a processor 203, a MIMO encoder 204, abeamformer/precoder 205, and a plurality of transmitters coupled to aplurality of antennas, respectively. Receiving device 211 comprisesmemory 212, a processor 213, a channel estimation module 214, asub-channel feedback module 215, and a plurality of receivers coupled toa plurality of antennas, respectively. SU-MIMO channel 221 is formed bythe plurality of transmitting antennas of transmitting station 201 andthe plurality of receiving antennas of receiving station 211. Thoseskilled in the art would realize that “antenna” is used in a logicalcontext, and may not necessarily be referred to as the physical antennastructure. SU-MIMO communication promises large gains for both channelcapacity and reliability, essentially via the use of spatial-time codesand transmit beamforming (diversity gain oriented) combined with spatialstream multiplexed transmission (rate maximization oriented). AlthoughSU-MIMO channel is illustrated in FIG. 2, the present invention is notlimited to SU-MIMO. In OFDMA systems, MU-MIMO over differentsub-channels may also be supported.

The various function modules may be implemented and configured bysoftware, firmware, hardware, and any combination thereof. The functionmodules, when executed by the processors (via program instructionscontained in the memory), interwork with each other to allow thetransmitting and receiving devices to perform certain embodiments of thepresent invention accordingly. For example, at the transmitter side,transmitting device 201 transmits a sounding signal to receiving device211. At the receiver side, receiving device 211 estimates the channelbased on the received sounding signal. The receiving device 211 thenfeedbacks the sub-channel SNR (signal-to-noise ratio) and CSI (channelstate information) to the transmitting device 201 via a feedback channel222. The feedback information comprises SNR/CSI information for a subsetof all the sub-channels based on a predefined condition to reduceoverhead.

FIG. 3 illustrates a novel feedback frame format in an IEEE 802.11wireless system. In the example of FIG. 3, a MAC frame 300 contains a HT(high throughput) control field, which further contains an MCS request(MRQ) subfield, an MCS sequence identifier (MSI) subfield, an MFBsequence identifier/LSB of Group ID (MFSI/GID-L) subfield, a VHT N_STS,MCS, BW and SNR feedback (MFB) subfield, an MSB of Group ID (GID-H)subfield, coding type of MFB response (Coding Type) subfield,transmission type of MFB response (FB Tx Type) subfield, unsolicited MFBsubfield, AC constraint subfield, and RDG/More PPDU subfield. The MFBsubfield 310 further contains a number of spatial streams (N_STS)subfield, an MCS subfield, a bandwidth (BW) subfield, and an SNRsubfield.

When MFB is requested, the MFB subfield contains the number of spatialstreams, modulation and coding scheme, data transmission rate,bandwidth, and SNR information. In the example of FIG. 3, the receivingdevice provides the MFB subfield feedback information for a subset ofsub-channels of a 160 MHz wide channel. The MFB subfield 310 containsSNR information for the corresponding sub-channels. Various solutionsfor sub-channel SNR feedback is now described below.

FIG. 4 illustrates transmission sub-channels in a wireless system andexamples of sub-channel SNR feedback. In the example of FIG. 4, the widechannel contains total eight transmission sub-channels in the system,indexed from 1 to 8. In a first solution, each STA feedback SNRs on allsub-channels, as depicted by option 1. If 8 bits are used for SNR oneach sub-channel, then only 64 bits needed for sub-channel SNR feedback.For MIMO system with more than one spatial stream, the STA may feedbackaverage SNR over all spatial streams on each sub-channel. Alternatively,the STA may feedback the sum throughput of all spatial streams on eachsub-channel. In a second solution, to further decrease the feedbackoverhead, each STA feedback the SNR followed by a sub-channel ID on thebest sub-channel only (e.g., sub-channel #3), as depicted by option 2.The system throughput under solution 2 will decrease as compared tosolution 1.

In a third solution, each STA feedback the sub-channel CSI with SNR overa predefined threshold. The threshold of each sub-channel can be a givennumber so that a particular MCS can be achieved. Each STA only feedbackthe indices of the sub-channels, as depicted by option 3. The STA canfeedback the sub-channel indices as well as the corresponding SNR. Thethreshold can also be determined by AP. For example, the AP decides thethreshold on each sub-channel based on the current max SNR of eachsub-channel or based on the SNR of the serving STA on each sub-channel.The AP can choose a smaller threshold to guarantee that at least one STAwill send a feedback to the AP on each sub-channel. In a fourthsolution, each STA feedback SNR on N best sub-channels or SNR on N worstsub-channels, as depicted by option 4, where N is a predefined number(e.g., N=3). Note that in all solutions, SNR can be replaced byrecommended MCS.

FIG. 5 illustrates the theoretical capacity in an OFDM/OFDMA wirelesssystem with SNR sub-channel feedback. As depicted in FIG. 5, simulationresults show that only feedback the SNR on the best sub-channel canalready achieve 80% throughput gain with 10 STAs and 95% throughput gainwith 20 STAs.

FIG. 6 illustrates SNR sub-channel feedback in OFDMA systems supportingMU-MIMO. For OFDMA systems supporting MU-MIMO on each sub-channel, themaximum system throughput for each sub-channel may be given by MU-MIMO.For example, it may be optimal to transmit on the strongest spatialstreams to different STAs. The transmitter needs more feedbackinformation to support MU-MIMO in a sub-channel. In order for the AP toselect the users, feedback of the average SNR over all spatial streamsor feedback of the sum throughput of all spatial streams may not beenough. Instead, the SNR of each spatial stream are needed. Thedirection of each spatial stream are also needed. The STA can choose tofeedback the information only on several strongest spatial streams. Thedirection of the spatial stream can be the right singular vector of thecorresponding spatial stream. The SNR and the direction of the spatialstream can also be replaced by compressed channel matrix. In the exampleof FIG. 6, for each sub-channel, the feedback information comprises theaverage SNR, plus the compressed channel matrix V for the strongest Ksubcarriers.

FIG. 7 illustrates a two-step SNR sub-channel feedback method in OFDMAMU-MIMO systems to further reduce feedback overhead. In the example ofFIG. 7, in step 1, the AP receives sub-channel SNR report from both STA1and STA2. For example, the SNR report includes the average SNR over allspatial streams or the sum throughput of all spatial streams for eachsub-channel. In step 2, based on the SNR report, the AP selects the STAsand sub-channels with good average SNR or high sum throughput andrequest detailed CSI feedback over those selected sub-channels. Thedetailed CSI feedback includes the compressed channel matrix of thesub-channels or per stream SNR and direction of the spatial streams ofthe sub-channels as explained in the previous paragraph.

In WLAN systems, the feedback overhead includes two parts: the CSIreporting overhead, and the channel contention overhead. With largenumber of STAs, the feedback overhead will be large. The above-proposedsub-channel SNR feedback design will dramatically decrease the CSIreporting overhead. However, channel contention overhead for CSIreporting can still be large. A new feedback scheme with low channelcontention overhead is thus desired.

FIG. 8 illustrates a novel acknowledgement (ACK) frame and a novel blockacknowledgement (BA) frame with piggybacked SNR sub-channel feedback. Inone novel aspect, the feedback information may be piggybacked within anACK or BA frame to reduce channel contention overhead. Since an STAoften needs to respond to AP with ACK or BA frame for received DL dataframes, by embedding the feedback information into the ACK frame or theBA frame, the additional channel contention for sending feedbackinformation can be omitted. In the example of FIG. 8, ACK frame 810comprises a frame control field, a duration, RA, a novel sub-channel FBelement, and a Frame checksum (FCS). Block ACK frame 820 comprises aframe control field, a duration, RA, TA, a BA control field, BAinformation, a novel sub-channel FB element, and a frame checksum (FCS).For example, the sub-channel FB element can have any of the formats asillustrated earlier with respect to FIG. 4.

FIG. 9 illustrates one embodiment of sending a separate SNR sub-channelfeedback frame after ACK/BA frame. As an alternative to piggybackedfeedback, an STA may send a separate feedback frame after the ACK/BAframe. In the example of FIG. 9, the AP first sends a channel-soundingframe 901, followed by a DL data frame 902 to the STA. The STA respondsan ACK/BA frame 911 to the AP. After a short inter-frame space (SIFS),the STA also sends a feedback frame 912 that contains sub-channel SNRfeedback information to the AP.

In some situation, an STA may not have chance to send ACK/BA frame tothe AP. Therefore, other mechanisms for sub-channel feedback shall alsobe provided. In one embodiment, unsolicited feedback mechanism may beused. The STA can initiate the sub-channel CSI feedback without APsolicitation, as depicted earlier for SAT3 in FIG. 1. The sub-channelfeedback information can be carried in a modified MFB element, asdepicted earlier in MFB 310 of MAC frame 300 in FIG. 3.

FIG. 10 illustrates another embodiment of SNR sub-channel feedback inresponse to polling. In the example of FIG. 10, AP first sends a pollingframe 1001 to a plurality of STAs. The polling frame contains a list ofSTA addresses of the polled STAs, e.g., STA1, STA2, and STA3. Uponreceiving the polling frame, STA1 sends a sub-channel FB frame 1011 backto the AP, STA2 sends a sub-channel FB frame 1021 back to the AP, andSTA3 sends a sub-channel FB frame 1031 back to the AP. The sub-channelFB frames contain SNR/CSI information of a subset of all thesub-channels.

FIG. 11 illustrates one embodiment of SNR sub-channel feedback afterpolling and OFDMA data transmission. In the example of FIG. 11, the APfirst sends a polling frame 1101 in OFDMA format, polling a list oftarget STAs for feedback information. The AP then sends an OFDMA dataframe 1102. The structure of the OFDMA data packet contains a legacypreamble, OFDMA data over various sub-channels, and LTFs for channelsounding. The OFDMA data frame is for certain STAs, e.g., STA1, STA2,and STA4. Upon receiving the OFDMA data frame, the corresponding STAssend OFDMA format ACK/BA frames back to the AP. After sending the ACK/BAframes, the target STAs listed in the polling frame send the CSI/SNRfeedback (FB) in the order specified in the address list (e.g., FB1,FB2, FB3, and FB4). Alternatively, the FB frames may be sent using OFDMAformat as well.

FIG. 12 illustrates another embodiment of piggybacked SNR sub-channelfeedback after OFDMA data transmission. In the example of FIG. 12, theAP only request STAs involved in the current OFDMA transmission tofeedback the sub-channel SNR. In this way, the SNR information can bepiggybacked in the ACK/BA frame. The AP does not need to send a separatepolling frame. Instead, the AP only needs a signaling bit to signal thetarget STAs for sub-channel SNR feedback indication. For example, the APcan use one reserved bit in HT control middle subfield for suchindication. As illustrated in FIG. 12, AP first sends OFDMA data frame1201 over various sub-channels followed by LTFs for channel sounding.The recipient STAs reply with OFDMA format ACK/BA frames back to the AP.For STAs with FB signaling indication, e.g., STA1, STA2, and STA4, theACK/BA frames are also embedded with sub-channel SNR feedbackinformation.

FIG. 13 is a flow chart of a method of transmitting SNR sub-channelfeedback in an OFDMA wireless communication system. In step 1301, awireless receiving device (STA) receives a radio signal from atransmitting device (AP) over a wide channel. The radio signal istransmitted over multiple sub-channels of the wide channel. In step1302, the STA estimates channel quality information based on thereceived radio signal for each sub-channel. In step 1303, the STA sendsfeedback information to the transmitting device. The feedbackinformation comprises the estimated channel quality information for aselected subset of sub-channels from the wide channel based on apredefined rule. The channel quality information comprises an averagesignal to noise ratio (SNR) of a selected sub-channel, or a sumthroughput of all spatial streams of the selected sub-channel. In oneembodiment, the selected subset of sub-channels is based on a predefinedchannel quality threshold. In another embodiment, the selected subset ofsub-channels is based on a predefined number of sub-channels having thebest channel quality. In yet another embodiment, the feedbackinformation is embedded within an ACK/BA frame or is carried in a frameimmediately subsequent to the ACK/BA frame.

FIG. 14 is a flow chart of a method of receiving SNR sub-channelfeedback in a wireless communication system. In step 1401, atransmitting device (AP) transmits a radio signal over a wide channel toa plurality of receiving devices (STAs). The radio signal is transmittedover multiple sub-channels of the wide channel. In step 1402, the APreceives feedback information from the STAs. The feedback informationcontains estimated channel quality information for a subset ofsub-channels from the wide channel for each STA. In step 1403, the APselects corresponding sub-channels for data transmission to the STAsbased on the feedback information. In one embodiment, the AP maydetermine a channel quality threshold such that at least one STA willsend a feedback to the AP on each sub-channel. In another embodiment,the AP may select the STAs with good average SNR/sum throughput andrequest for detailed CSI feedback for MU-MIMO data transmission.

Although the present invention has been described in connection withcertain specific embodiments for instructional purposes, the presentinvention is not limited thereto. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

What is claimed is:
 1. A method comprising: (a) receiving a radio signaltransmitted from a transmitting device over a wide channel by areceiving device in an Orthogonal Frequency Division Multiple Access(OFDMA) wireless system, wherein the radio signal is transmitted overmultiple sub-channels of the wide channel; (b) estimating channelquality information based on the received radio signal for eachsub-channel; (c) sending feedback information to the transmittingdevice, wherein the feedback information contains the estimated channelquality information for a selected subset of sub-channels from the widechannel based on a predefined rule; receiving a channel stateinformation (CSI) feedback request for a Multi-user multiple-input andmultiple-output (MU-MIMO) sub-channel; and sending a channel matrixreport of the MU-MIMO sub-channel; wherein the selected subset ofsub-channels is based on a predefined channel quality threshold, or theselected subset of sub-channels is based on a predefined number ofsub-channels having the best channel quality.
 2. The method of claim 1,wherein the channel quality information comprises an average signal tonoise ratio (SNR) of a selected sub-channel.
 3. The method of claim 1,wherein the channel quality information comprises a sum throughput ofall spatial streams in a selected sub-channel.
 4. The method of claim 1,wherein the radio signal contains both OFDMA data field for multiplesub-channels and multiple training fields for channel sounding.
 5. Themethod of claim 1, wherein the feedback information is carried in anacknowledgement (ACK) frame or a block acknowledgement (BA) frame. 6.The method of claim 1, wherein the feedback information is carried in aframe immediately subsequent to an ACK frame or a BA frame.
 7. Awireless station, comprising: a receiver that receives a radio signaltransmitted from a transmitting device over a wide channel in anOrthogonal Frequency Division Multiple Access (OFDMA) wireless system,wherein the radio signal is transmitted over multiple sub-channels ofthe wide channel; a channel estimation module that estimates channelquality information based on the received radio signal for eachsub-channel; a transmitter that sends feedback information to thetransmitting device, wherein the feedback information contains theestimated channel quality information for a selected subset ofsub-channels from the wide channel based on a predefined rule; thereceiver that receives a channel state information (CSI) feedbackrequest for a Multi-user multiple-input and multiple-output (MU-MIMO)sub-channel; and the transmitter that sends a channel matrix report ofthe MU-MIMO sub-channel; wherein the selected subset of sub-channels isbased on a predefined channel quality threshold, or the selected subsetof sub-channels is based on a predefined number of sub-channels havingthe best channel quality.
 8. The station of claim 7, wherein the channelquality information comprises an average signal to noise ratio (SNR) ofa selected sub-channel.
 9. The station of claim 7, wherein the feedbackinformation is carried in an acknowledgement (ACK) frame or a blockacknowledgement (BA) frame.
 10. The station of claim 7, wherein thefeedback information is carried in a frame immediately subsequent to anACK frame or a BA frame.
 11. A method, comprising: (a) transmitting aradio signal from a transmitting device to a plurality of receivingdevices over a wide channel in an Orthogonal Frequency Division MultipleAccess (OFDMA) wireless system, wherein the radio signal is transmittedover multiple sub-channels of the wide channel; (b) receiving feedbackinformation from the plurality of receiving devices, wherein thefeedback information contains the estimated channel quality informationfor a subset of sub-channels from the wide channel for each receivingdevice; (c) selecting corresponding sub-channels for data transmissionto the plurality of receiving devices based on the feedback information;determining a channel quality threshold such that the channel quality ofeach sub-channels from the subset of sub-channels is above thethreshold, or the subset of sub-channels comprises a predeterminednumber of sub-channels having the best channel quality; transmitting achannel state information (CSI) feedback request for a Multi-usermultiple-input and multiple-output (MU-MIMO) sub-channel; and receivinga channel matrix report of the MU-MIMO sub-channel.
 12. The method ofclaim 11, the channel quality information comprises an average signal tonoise ratio (SNR) of a selected sub-channel.
 13. The method of claim 11,wherein the radio signal contains both OFDMA data field for multiplesub-channels and multiple training fields for channel sounding.
 14. Themethod of claim 11, wherein the feedback information is carried in anacknowledgement (ACK) frame or a block acknowledgement (BA) frame. 15.The method of claim 11, wherein the feedback information is carried in aframe immediately subsequent to an ACK frame or a BA frame.